1. Field of the Invention
This invention relates generally to a method for treating process waste streams and, more particularly, the invention is directed to the treatment of cattle, fish, and poultry process waste streams by the sequential addition of natural flocculants.
2. Description of Related Technology
Conventional methods for treating process waste streams by flocculation typically require the use of one or more high molecular weight synthetic flocculants. The resultant flocculent mass is recovered by mechanical dewatering methods such as vacuum filtration, centrifugation, press filtration, or flotation. The desired goal of the prior flocculation methods is to obtain large, rigid floc with low moisture content which will result in excellent dewatering performance.
More specifically, one approach has been a two-stage flocculation method in which a first high molecular weight synthetic flocculant of an ionic charge opposite to that of the waste stream being treated is added to the waste stream to neutralize the ionic charge thereof. A second high molecular weight synthetic flocculant of an ionic charge opposite to that of the first flocculant is next added to the system to form giant stout floc. Recovery is effected by conventional dewatering methods.
In a second approach a polymeric synthetic flocculant of an ionic charge opposite to that of the waste stream being treated is added to the stream in an amount sufficient to barely reverse the ionic charge or zeta-potential of the waste stream to a point where numerous small floc are present. A second polymeric synthetic flocculant of an ionic charge opposite to that of the first flocculant is then added in an amount sufficient to again reverse the zeta-potential of the system and to form large floe of a minimum diameter of at least 0.1 mm. See U.S. Pat. No. 3,617,568.
The conventionally used flocculants in these prior methods are synthetic flocculants because they produce strongly bonded floc which are relatively large and rigid and are, therefore, desirable for the subsequent mechanical dewatering operation. The most commonly used synthetic flocculants are polyacrylamides and their derivatives. Polyacrylamides are preferred for use as at least one of the flocculants in the prior processes because their chemical structure contains many double bonds which will result in relatively strong, rigid floc formation.
Attempts have been made to use natural flocculants as part of prior flocculation methods. One method includes the step of adding, in a first stage, a first high molecular weight flocculant with agitation to neutralize (rather than reverse) the ionic charge of the waste stream being treated. However, the first stage agitation intensity must be greater than that ordinarily employed for obtaining flocculation so as to form small size floc no greater than 2 mm in diameter, or to form no floc at all. A second high molecular weight flocculant of a polarity (ionic charge) opposite to the first flocculant is next added with milder agitation until large, rigid, non-sticky flocs are formed. One of the flocculants can be a natural high molecular weight flocculant while the other flocculant is typically a high molecular weight synthetic flocculant, preferably a polyacrylamide or a derivative. The resulting floc is then subjected to a dewatering operation. An important aspect of this process is the neutralization of the waste stream to obtain flocs less than 2 mm in size after the first flocculant is added under high speed agitation and then to add a second flocculant under milder agitation conditions to form flocs larger than those formed after the addition of the first flocculant.
Alginate and other crude algal compositions have been shown to be effective natural flocculants for some waste waters when used alone. The drawbacks to using alginates is that they form weak chemical bonds with the components in the waste stream and pH adjustments to the waste stream are necessary. Moreover, complete flocculation of the suspended solids, particularly those containing blood, is not consistently obtained because the process is pH sensitive.
Shortcomings of these prior processes include difficulty in treating a waste stream containing fatty acids, proteins, grease, or oils, such as are found in most animal and food process waste streams, without using at least one synthetic flocculant, because the floc which is formed in these waste streams can be sticky and/or weakly bonded. It is believed that because natural flocculants contain fewer double bonds than high molecular weight synthetic flocculants, weaker or less bonding with the suspended solids will occur. Consequently, insufficient conditioning will result in little or no floc formation or weakly bonded floc making subsequent recovery difficult. Merely adding a first flocculant of a charge opposite to that of the waste stream being treated to reverse or neutralize the ionic charge or zeta potential of the waste stream and next adding a second flocculant with an ionic charge opposite to the first flocculant will not necessarily provide the desired flocculation when using all natural flocculants. Thus, in the prior methods whenever a natural flocculant is used, an additional high molecular weight synthetic flocculant is also selected in order to acquire the desired flocculent mass for subsequent recovery.
A major drawback to using synthetic flocculants, especially the widely used polyacrylamides or derivatives, is that they can be highly toxic to the environment. As of this date, the Food and Drug Administration (FDA) has refused to approve food additive petitions for the use of polyacrylamides in the treatment of food and poultry processing plant waste streams with the recycling of the captured food solids, fats and greases to animal feed. Consequently, there are increasing concerns regarding the use and disposal of polyacrylamides by the EPA. Additionally, restrictions may be placed on industrial process and sewage wastewaters that are being discharged into municipal sewage systems and sanitary districts and fines may be imposed against the producers of industrial waste exceeding these limits. National Pollution Discharge Elimination System (NPDES) discharge permits place limits on fats, oils and greases (FOG), biological oxygen demand (BOD), chemical oxygen demand (COD), and suspended solids (SS) found in these waste streams.
Moreover, the polyacrylamides and derivatives typically used in treating process waste streams are derived from hydrocarbon monomer polymerization of acrylamide and acrylic acid to create long molecular weight chain polymers. These polymers are present in the end-product made from the prior processes and may contain free (re: unpolymerized) acrylamide that is known to be highly toxic, carcinogenic, and not biodegradable. Studies obtained from independent laboratory testing conducted by expert scientific researchers indicate that these synthetic polymers and associated non-polymerized monomers show positive potential health risks to animals and man when used in animal feed or feed supplements. Thus, the uses for the recovered end-product from these prior processes can be highly restrictive and disposal is limited and expensive. As a result, there is a growing need for a safe and effective flocculation method for treating process waste streams that does not contaminate the environment but will provide for a safe use for the recovered end-product.